Data storage subsystems may comprise various forms of data storage to initially store data, such as DASD fast write data from host systems, and subsequently to destage the data to more permanent data storage, such as DASD or disk drives. In one example, the data storage subsystem may comprise a pair of clusters, each with cache data storage which is volatile, and non-volatile data storage. The pair of clusters provide backup to the data in that one set of dual mode DASD fast write data of a logical subsystem is stored in the cache data storage of a first cluster and in the non-volatile data storage of the second cluster, and dual mode DASD fast write data of another logical subsystem is stored in the cache data storage of the second cluster and in the non-volatile data storage of the first cluster. As an example, in dual cluster mode, even logical subsystems use the cache in the left cluster and the non-volatile storage in the right, and odd logical subsystems use the cache in the right cluster and the non-volatile storage in the left.
As the result, all of the dual mode DASD fast write data is protected, for example against a power failure or reboot event, by being stored in a non-volatile store in one of the clusters.
In the event of a failure of one of the clusters, a failover is executed to the other cluster, and all of the dual mode DASD fast write data is available on the other cluster, with one set of data stored in the non-volatile data storage and another set of data stored in the cache data storage.
Thus, in response to a failover of one of the pair of clusters to the local cluster, the dual mode DASD fast write data that is stored in local cache storage and that was known to be stored in non-volatile storage in the other cluster, becomes the only copy of the data. The cache data storage is volatile, leaving the only copy of the data vulnerable.